1. Field of Invention
The present invention relates to methods and apparatus for seismic exploration.
2. Description of Prior Art
In the prior art, geophone arrays for land seismic operations were positioned, or laid out, by individually placing or "planting" each geophone by hand. Typically, the geophones were clustered in groups at a plurality of stations, each geophone at a station being connected to auxiliary wiring known in the art as "strings" or "pigtails." Multi-conductor geophone cables were used to transmit the signals sensed by the geophones during seismic exploration to a recording apparatus. The cables and auxiliary wiring were positioned by men in the field or mechanically reeled from vehicle-mounted equipment such as storage reels. Consequently, placing the geophones and associated wiring and cables in position for seismic exploration and retrieving same when moving to a new location after completion of operations was costly, laborious, and time-consuming.
In marine seismic exploration operations, ballasted streamer cables having pressure type geophones therewith were drawn behind a marine vessel. The water medium permitted smooth pulling and towing operations with the streamer cables and permitted easy and ready acoustic coupling of the seismic energy with the pressure geophones.
Previous attempts in seismic exploration to push, pull, or tow a land geophone cable have been unsatisfactory for several reasons. Such geophone cables have heretofore utilized a long conductor cable having a protective plastic or rubber exterior covering and having velocity geophones mounted by gimbals in covered housings in the cable, or in housings coupled to the cable by coupling wire. The gimbal mountings were required to maintain the geophones upright to provide accurate sensing of seismic signals.
The prior art geophone cables were heavy and cut into the earth's surface or ground during movement and use and were thus difficult to pull, particularly if of substantial length. Also, such cables were expensive and subject to wear. Further, and more importantly, the gimbal mountings prevented effective decoupling and isolation of the geophones from vibrations and the like induced in or sensed by the taut towing cables during seismic exploration, reducing the sensitivity and accuracy of the geophones. When the cables and geophones were used on uneven ground surfaces, difficulties in decoupling and isolation, and problems in achieving a good ground coupling between the geophone and the ground surface were further increased.